BACKGROUND: Aspiration pneumonitis is one of the most severe anesthetic complications. The severity of this complication is associated with the acidic nature of aspirated gastric juice. H2 receptor antagonists inhibit gastric acid secretion and elevate the gastric juice pH. The purpose of this study was to compare the effects of preoperative night cimetidine, ranitidine and famotidine medications on gastric volume and pH. METHODS: We studied 84 healthy patients, ASA physical status I or II, who were each randomly allocated to one of 4 groups. Group I ingested only 200 ml of milk up to 8 10 hours prior to anesthesia. Groups II, III and IV ingested 200 ml of milk and 400 mg of cimetidine, 150 mg of ranitidine and 20 mg of famotidine, respectively, on the evening before elective surgery. After the induction of anesthesia, gastric juice was collected with 18 French Salem sump tube, and gastric volume and pH were measured. RESULTS: There were no significant differences in gastric juice volume among the numbers of the 4 groups. However, Group IV (famotidine) showed a significant high pH, compared with the other groups. CONCLUSIONS: Famotidine 20 mg, which is premedicated on the evening before elective surgery, is more effective in preventing aspiration pneumonitis than cimetidine 400 mg and ranitidine 150 mg.